1. Field of the Invention
The present invention relates to the preparation of a purified basic aluminum chlorosulfate.
2. Description of the Prior Art
Basic aluminum chlorosulfates are materials well known to this art which are especially useful as coagulating agents in the treatment of aqueous media.
Thus, in published European Patent Application No. 218,487 a process is described for the preparation of such chlorosulfates having the indicated utility. This process is characterized by the following sequence of stages:
(i) initially, a slurry is formed of a mixture of calcium chloride and calcium carbonate;
(ii) the slurry is then reacted (continuously or discontinuously) with aluminum sulfate; and
(iii) finally, the reaction mixture produced in this manner is filtered, whereby a calcium sulfate filter cake and a filtrate containing the desired basic aluminum chlorosulfate is obtained.
When employed as coagulating agents for the treatment of impure aqueous media (for example, effluents, residual waters, waste waters, drinking waters, etc.), these compounds have been found to display a remarkable performance relative to both their stability and their coagulating and flocculating capacity, and result in an especially low degree of residual aluminum in the waters treated, over a wide range of pH values.
However, as indicated in the aforesaid '487 application, the basic aluminum chlorosulfate solutions obtained after the filtration of the reaction medium may display, in certain cases, a color varying from a very deep brown to a dark yellow, and a turbidity that is very much higher than one hundred NTU units.
Such turbidity, which may militate against the use of these solutions, is due to a particularly high proportion of solids in solution, specifically relative to gypsum fines which pass through the filter in the filtering stage, and it is likely that the strong coloration of the resulting solutions emanates from colored impurities, based in particular on iron and humic acid substances, with the nature and amounts of colorants in the final product certainly varying as a function of the quality of the raw materials employed.
To eliminate this disadvantage, it is proposed in the '487 application to add, whether together or separately, in at least one of the stages of the process described, but always prior to filtration, at least one adsorbent agent and at least one flocculating polyelectrolyte of the nonionic or cationic type. The adsorbent agent particularly recommended is activated charcoal.
However, such an improvement nevertheless has several serious disadvantages.
On the one hand, it makes it necessary to use activated charcoal, which colors and pollutes the gypsum cake formed in the filtering stage; it then becomes impossible to market the large amounts of recovered gypsum, particularly to the paper industry.
On the other hand, it requires a very careful filtering stage to ensure that the activated charcoal does not pass through the filters to color the final solution of the basic aluminum chlorosulfate. This second problem is generally limited by increasing the amounts of the polyelectrolyte introduced, but obviously to the detriment of the economy of the process.
Finally, it is observed that the turbidity of the final solutions of the basic aluminum chlorosulfate upon completion of the process, already low (on the order of about ten NTU units), cannot be further significantly reduced, even after filtering and/or extensive decantation.